PROJECT SUMMARY Subjective perceptions of stress mediate the effects of acute and chronic stressors on long-term psychological outcomes, and thus represent an important risk factor for disorders such as PTSD and depression. The proposed project will fill a knowledge gap by identifying novel putative brain mechanisms of perceived stress, thus allowing for better identification of at-risk individuals and the development of treatment targets for interventions that seek to enhance stress resilience. This Career Development Award will provide the candidate with training in advanced structural and diffusion-weighted MRI methods and didactic and mentored training in the fundamentals of clinical trials research. This new training will be leveraged to test novel hypotheses regarding hippocampal contributions to perceived stress, and modification of these mechanisms by a mindfulness intervention. In a sample of 300 individuals from the community, Study 1 will investigate relationships between perceived stress and pattern separation behavior, a hippocampal-dependent process that allows for fine-grained discrimination of safe vs. threatening contexts, and which has been theoretically linked to over-generalization of fear and negative affect. Using high-resolution structural imaging and multi-atlas segmentation of the hippocampus, Aim 1 will investigate relationships between pattern separation behavior, hippocampal subfield volumes, and perceptions of daily life stressors and acute laboratory stressors. In Aim 2, advanced diffusion-weighted imaging methods will be used to gain insight into the density and orientation of hippocampal neurites (axons and dendrites). This will allow for a novel investigation of the microstructural correlates of subjectively perceived stress, hypothesized to include reduced neurite density within the hippocampus and less coherent orientation of white matter tracts linking the hippocampus and medial prefrontal cortex. In Study 2, 80 participants will be randomized to a wait-list control group or mindfulness-based stress reduction (MBSR), which is known to reduce subjective perceptions of stress. Aim 3 will test whether MBSR participants demonstrate improvements in pattern separation behavior, hippocampal volume, and hippocampal microstructure, and whether these hippocampal changes are associated with expected reductions in perceived stress. Aims 1-2 will be facilitated by new training in advanced structural imaging of the hippocampus and advanced diffusion-weighted imaging methods, supported by a training team with extensive expertise in these domains and made possible by the rich resources of a world-class brain imaging center. Aim 3 will be supported by didactic, experiential, and mentored training in the fundamentals of clinical research through the NIH-funded Institute for Clinical and Translational Research (ICTR). Collectively, the research and training aims of this project seek to illuminate novel neurobiological mechanisms of perceived stress and provide new training in advanced neuroimaging methods and clinical trials methodology. This award will enable the candidate to conduct future intervention studies aimed at preventing the development of stress-related disorders by engaging these neurobiological targets.